I presonally don't feel that any horse should naturall be a narrow fitting - is he hollow either side of his withers? Sitting too low at the front does not always mean that the tree width is wrong - the tree may be too flat, it may not have deep enough panels etc. If he is hollow you may want to fit wider with padding. Do you have a good fitter you can work with? I fit a deeper seated WH model to those shaped backs so they are out there.
Ideal make SOME Fyldes, the rest are totally different in design and quality.